Multilayer base heterojunction bipolar transistor

ABSTRACT

This is a p-n junction device and the device comprises: a substrate 10 composed of a semiconductor material; a heavily doped n type sub-collector layer 14 over the substrate; a n type collector layer 16 over the sub-collector layer; a heavily doped p type first base layer 18, over the collector layer; a p type second base layer 20, substantially thinner than the first base layer, over the first base layer, with the second base layer being less heavily doped than the first base layer; and a n type emitter layer 24 over the second base layer, whereby, the second base layer serves as a diffusion barrier between the base and the emitter. Other devices and methods are also disclosed.

NOTICE

(©) Copyright, *M* Texas Instruments Incorporated 1991. A portion of thedisclosure of this patent document contains material which is subject tocopyright and mask work protection. The copyright and mask work ownerhas no objection to the facsimile reproduction by anyone of the patentdocument or the patent disclosure, as it appears in the Patent andTrademark Office patent file or records, but otherwise reserves allcopyright and mask work rights whatsoever.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following coassigned patent applications are hereby incorporatedherein by reference:

    ______________________________________                                        Serial No.    Filing Date                                                                              TI Case No.                                          ______________________________________                                        07/693490     April 30, 1991                                                                           TI-15800                                             ______________________________________                                    

FIELD OF THE INVENTION

This invention generally relates to p-n junction diffusion barriers.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is describedin connection with the diffusion of dopants in the base layer of aheterojunction bipolar transistor (HBT), as an example.

Heretofore, in this field, it has been the object of device designers todevelop HBTs to provide signal gain at microwave frequencies. Anecessary consequence of this design goal is the formation of astructure that exhibits a low series base resistance. The speed of HBTsdepends strongly on the device base resistance, which must be kept lowto minimize resistive parasitics of the input.

Self-aligned emitter-base contact fabrication techniques are often usedto minimize the series base resistance by lowering the distance betweenthese two contacts. The lowering of the base sheet resistance is,however, the most effective way to achieve a reduction in all componentsof the base resistance (series resistance, contact resistance, andaccess resistance).

Low base sheet resistance in npn HBTs with base layers made fromcompound semiconductor materials, such as GaAs, has been achieved byheavily doping the base layer with Be or Zn. Base doping concentrationin excess of 1×10¹⁹ cm⁻³ is needed for microwave performance. Suchheavily doped base layers, however, are not stable under hightemperature growth or under high temperature and bias operatingconditions. Degenerately doped base layers can act as diffusion sourcesunder these type of stress conditions and p-type dopant can diffuse intothe emitter and/or the collector layer irreversibly changing the deviceparameters. The presence of a heavily doped n-type layer close to thep-type doped layer also causes diffusion enhancement during layer growthat elevated temperatures (typical growth temperatures in MOCVD is600°-800° C.).

SUMMARY OF THE INVENTION

This is a p-n junction device. The device comprises: a substratecomposed of a semiconductor material; a heavily doped n typesub-collector layer over the substrate; a n type collector layer overthe sub-collector layer; a heavily doped p type first base layer, overthe collector layer; a p type second base layer, substantially thinnerthan the first base layer, over the first base layer, with the secondbase layer being less heavily doped than the first base layer; and a ntype emitter layer over the second base layer, whereby, the second baselayer serves as a diffusion barrier between the base and the emitter.

Preferably, the device has a buffer layer over the substrate and underthe sub-collector layer; a third base layer over the collector layer andunder the first base layer, with the third base layer being less heavilydoped than the first base layer, whereby the third base layer serves asa diffusion barrier between the base and the collector; a n type emitterstop layer over the second base layer and under the emitter layer, agrading layer over the emitter layer; a heavily doped n type firstcontact layer over the emitter layer or over a grading layer over theemitter layer; a n type second contact layer over the first contactlayer, with the second contact layer being more heavily doped than thefirst contact layer; the first contact layer, second contact layer,first base layer, second base layer, sub-collector layer and collectorlayer are GaAs; and the emitter layer is AlGaAs.

This is also a method of forming a p-n junction device, the methodcomprises; forming a substrate composed of a semiconductor material;forming a heavily doped n type sub-collector layer over the substrate;forming a n type collector layer over the sub-collector layer; forming aheavily doped p type first base layer over the collector layer; forminga p type second base layer, substantially thinner than the first baselayer, over the first base layer, with the second base layer being lessheavily doped than the first base layer; forming a n type emitter layerover the second base layer, whereby, the second base layer serves as adiffusion barrier between the base and the emitter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1a-1b are cross-sectional views, at different processing stages, ofa preferred embodiment of this invention;

FIG. 2 is a more detailed view of a section of FIG. 1b;

FIG. 3 is a doping profile of a preferred embodiment of this invention.

Corresponding numerals and symbols in the different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1a, the layers of a preferred embodiment heterojunction bipolartransistor (HBT) are shown, which are formed epitaxially. The layers aresubsequently etched and contact is made, to form the device shown inFIG. 1b. FIG. 2 shows more detail of a portion of FIG. 1b (layers22-30). Refer to FIGS. 1a-1b and FIG. 2 in the following discussion ofthe preferred embodiment HBT. The preferred doping levels andthicknesses stated in the description below are approximate values only.

The device is on a substrate 10 composed of a semi-insulatingsemiconductor material, such as undoped GaAs. A buffer layer 12,typically of undoped GaAs or AlGaAs, is epitaxially formed over thesubstrate 10. A sub-collector layer 14 of n+ GaAs is epitaxially formedover the buffer layer 12. The doping of the sub-collector layer 14 ispreferably 2×10.sup.˜ cm⁻³,and the thickness is preferably 1.0 um. Acollector layer 16 of n- to n type GaAs is epitaxially formed over thesub-collector layer 14. The doping of the collector layer 16 ispreferably 2×10¹⁶ cm⁻³ and the thickness is preferably 1.0 um. Thedopant preferably used for n type doping, in this device, is Si.

The base region is formed over the collector layer 16 in two layers,with an optional third layer. The first base layer 18, of heavily (p++)doped p type GaAs material, typically doped 1-5×10¹⁹ cm⁻³, forms thebulk of the base of the transistor. The first base layer 18 ispreferably 1000-1500Å thick. This layer provides low sheet resistivityand low contact resistance. A thin, preferably 100-500Å thick, secondbase layer 20, of p+ GaAs, is epitaxially formed over the first baselayer 18 and serves as a diffusion barrier for the first base layer. Thesecond base layer doping is lighter than the doping of the first baselayer, preferably 1-2×10¹⁸ cm⁻³. The dopant preferably used for p typedoping, in this device, is Zn. The thicknesses and doping concentrationsof each layer is chosen considering the diffusion characteristics ofeach dopant. Generally, a layer of 100-500 Å is needed to effectivelystop the diffusion of dopants to the p-n junctions. The concentration ofdopant in these layers need to be low enough for minimal dopantoutdiffusion (i.e. less than 5×10.sup.˜ cm⁻³ for Zn) while high enoughto avoid significantly increased series resistance. The differencebetween the doping levels of these layers should also be minimized toavoid backdiffusion of minority carriers due to doping concentrationprofile induced reverse electrical field in the base. A third baselayer, with the same doping and thickness as the second base layer 20,may be added between the collector layer 16 and the first base layer 18to prevent diffusion into the collector during growth.

An optional n to n+ GaAs emitter stop layer 22 can be epitaxially formedover the second base layer. This layer 22 may be doped at 1×10.sup.˜cm⁻³ and 100 Å thick. The emitter stop layer 22 serves as a barrierbetween the emitter and the base of the device further isolating the Znto the base layers, therefore keeping the emitter n type, as needed forproper device performance. The emitter layer 24, of n type AlGaAs (thewide bandgap of which provides the heterojunction effect), isepitaxially formed over the emitter stop layer 22 (or over layer 20, iflayer 22 is not used), with a preferred doping of 2×10.sup.∫ cm⁻³ and apreferred thickness of 1000 Å. A thin grading layer 26 is epitaxiallyformed over the emitter layer 24, in which the concentration of Algradually diminishes (which provides a smooth bandgap transition fromAlGaAs to GaAs). A first contact layer 28 is formed over the gradinglayer 26 of n+ GaAs. This layer's doping concentration and thickness arepreferably 1×10.sup.˜ cm⁻³ and 1000 Å, respectively. A second contactlayer 30, more highly doped than the first contact layer 28 (whichprovides a suitable surface for deposition of emitter metallization), isepitaxially formed over the first contact layer 28. The second contactlayer is n+ GaAs with preferable doping of 5×10 cm⁻³ and thickness of1500 Å. The first contact layer 28 is used as a spacer between the firstbase layer 18 and the second contact layer 30. The use of this layerlowers the built-in electric field at the emitter-base junction, whichis responsible for diffusion enhancement during epitaxial layer growth.

After epitaxial formation of the desired layers, the structure isanisotropically etched, preferably using a process such as reactive ionetching (RIE) in BCl₃ plasma, to form the desired structure, such as theone shown in FIG. 1b. At this point, contact formation may be done. Theetching exposes the sub-collector layer 14 and the first base layer 18.If desired, the collector layer 16 may be left unetched on thesub-collector layer 14 and ion implantation may be used to contact thesub-collector layer 14 through the collector layer 16. The emittercontact 34, and the collector contacts 36 are preferably formed oflayered AuGe/Ni/Au at thicknesses of 500 Å,140 Å, and 4000 Å,respectively. The base contacts 32 are preferably layered Ti/Pt/Au atthicknesses of 500 Å, 250 Å, and 1500 Å, respectively.

In Table 1, below, the preferred ranges for both doping and thicknessesof the different layers of the device may be found. The preferred valuesmay be found in the discussion above. A graphical representation of thetypical doping levels of the device is shown in FIG. 3, and may be usedfor further clarification.

The described preferred embodiment of the present invention has theadvantage that an HBT can be formed with low base resistance, andtherefore high operating frequency, by the use of the heavily Zinc dopedfirst base layer 18. If not for the inclusion of the less heavily dopedsecond base layer 20, Zinc diffusion into the emitter 24, driven by heatdissipated by the device and the forward biased base-emitter p-njunction, would degrade the device performance irreversibly over itsoperating lifetime, producing unstable and unpredictable deviceperformance.

A preferred embodiment has been described in detail hereinabove. It isto be understood that the scope of the invention also comprehendsembodiments different from those described, yet within the scope of theclaims. For example, while the p type dopant is referred to as Zn, itcould also be materials such as C, Be, Mg, or Mn, and the n type dopant,while preferably silicon, could be a material such as S, or Se.Similarly, the AlGaAs emitter could be any appropriate wide bandgapmaterial such as InGaP and the GaAs could be replaced with a materialsuch as InGaAs or InP. The second base layer 20 can be used in relationto any two semiconductor layers where it is desired to control thediffusion of dopants from one layer into the other, for example, in anyp-n junction device (such as diodes, transistors, lasers, etc).

Words of inclusion are to be interpreted as nonexhaustive in consideringthe scope of the invention.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

                  TABLE 1                                                         ______________________________________                                                       TYPICAL      TYPICAL                                                          DOPING       THICKNESS                                         LAYER          RANGES (cm.sup.-3)                                                                         RANGES (um)                                       ______________________________________                                        SUB-COLLECTOR 14                                                                              1-4 × 10.sup.18                                                                     0.5-1.5                                           COLLECTOR 16   2-10 × 10.sup.16                                                                     0.3-3.0                                           FIRST BASE 18   1-7 × 10.sup.19                                                                     0.05-0.15                                         SECOND BASE 20  1-2 × 10.sup.18                                                                     0.01-0.05                                         EMITTER STOP 22                                                                              5-10 × 10.sup.17                                                                     0.006-0.015                                       EMITTER 24     1-10 × 10.sup.17                                                                     0.05-0.2                                          GRADING 26     3-10 × 10.sup.17                                                                     0.03-0.06                                         FIRST CONTACT 28                                                                              1-3 × 10.sup.18                                                                     0.1-0.2                                           SECOND CONTACT 30                                                                             3-6 × 10.sup.18                                                                     0.1-0.2                                           ______________________________________                                    

I claim:
 1. A p-n junction device, said device comprising:a. a substratecomposed of a semiconductor material; b. a heavily doped n typesub-collector layer over said substrate; c. a n type collector layerover said sub-collector layer; d. a heavily doped p type first baselayer, over said collector layer; e. a p type second base layer,substantially thinner than said first base layer, over said first baselayer, with said second base layer being less heavily doped than saidfirst base layer and with said doping of said second base layer beingessentially uniform: f. a n type emitter layer over said second baselayer, whereby, said second base layer serves as a diffusion barrierbetween said base and said emitter.
 2. The device of claim 1, whereinsaid device has a buffer layer over said substrate and under saidsub-collector layer.
 3. The device of claim 1, wherein said device has athird base layer over said collector layer and under said first baselayer, with said third base layer being less heavily doped than saidfirst base layer, and with said doping of said third base layer beingessentially uniform, whereby said third base layer serves as a diffusionbarrier between said base and said collector.
 4. The device of claim 1,wherein said device has a n type emitter stop layer over said secondbase layer and under said emitter layer.
 5. The device of claim 1,wherein said device has a grading layer over said emitter layer.
 6. Thedevice of claim 1, wherein said device has a heavily doped n type firstcontact layer over said emitter layer or over a grading layer over saidemitter layer.
 7. The device of claim 6, wherein said device has a ntype second contact layer over said first contact layer, with saidsecond contact layer being more heavily doped than said first contactlayer.
 8. The device of claim 7, wherein said first contact layer,second contact layer, first base layer, second base layer, sub-collectorlayer and collector layer are GaAs.
 9. The device of claim 1, whereinsaid emitter layer is AlGaAs.
 10. The device of claim 1, wherein saiddevice is a heterojunction bipolar transistor.